This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to a Japanese Patent Application 2001-153181, filed on May 22, 2001, the entire content of which is incorporated herein by reference.
This invention generally relates to a method for driving a bridge circuit for driving an inductance load. More particularly, this invention pertains to a bridge circuit driving method being for appropriately driving a bridge circuit for driving an actuator of an active type vibration control system.
Bridge circuits for driving an inductance load have been employed for various uses. A coil which forms an actuator of an active type vibration control system represents the inductance load. The active type vibration control system is disclosed, for example in a Japanese Laid-open Patent Publication (Kokai) No. 8(1996)-137556 and in another Japanese Laid-open Patent Publication (Kokai) No. 10(1998)-318326. Each active type vibration control system is applied to an automobile engine mount and offsets vibration of a vibration protecting object by applying an exciting force thereto. Further, a Japanese Laid-open Patent Publication (Kokai) No. 8(1996)-219227 discloses a bridge circuit including the coil forming the actuator of the active type vibration control system and a control means for controlling the bridge circuit.
The active type vibration control system disclosed in the Japanese Laid-open Patent Publication (Kokai) No. 8(1996)-219227 is provided with the bridge circuit parallelly connecting two sets of two switching elements. The two switching elements of the respective sets are connected to each other in series between a power supply and a ground. The coil forming the actuator is bridge-connected between connecting points of the switching elements of the respective sets. The active type vibration control system is further provided with a controller for comparing a detected value of an electric current flowing the coil with a set value for setting the electric current flowing the coil, for controlling the switching elements based upon the comparison result therebetween and an electric current polarity switch signal for switching the polarity of the electric current flowing the coil, and for controlling the polarity of the electric current flowing the coil lead wire and the electric current value thereof.
Generally, a bridge circuit for driving the inductance load such as the actuator having the coil is provided with a flywheel diode arranged in parallel with each switching element. The bridge circuit disclosed in the Japanese Laid-open Patent Publication (Kokai) No. 8(1996)-219227 is not described in detail with respect to the above structure. For example, energy is accumulated in the inductance of a load circuit in response to switching the polarity of the power source by the switching elements. The flywheel diode is hence provided for each switching element for effectively protect each switching element from being damaged due to the energy accumulated in the inductance. However, each switching element and each flywheel diode may not be operated following a desired mode depending on a driving signal supplied to each switching element.
For example, according to the active type vibration control system, a pulse-width modulation (PWM) can be applied to a control signal with a sine wave for generating vibration with an opposite phase relative to vibration of a controlled object. Meanwhile, the electric current may be deformed due to occurrence of counter electromotive force when directions (or polarities (positive or negative)) of a voltage and an electric current with respect to a voltagexe2x80x94current phase characteristics are not identical to each other. In such a case, undesired vibration may occur due to harmonic content. The deformation of the electric current is described later with reference to FIG. 10 and FIG. 11.
According to the Japanese Laid-open Patent Publication (Kokai) No. 8(1996)-219227, the electric current becomes the sine wave by an electric current feedback control. However, the voltage is alternately required to be reversed within a range in which the directions (or the polarities) of the voltage and the electric current with respect to the voltagexe2x80x94current phase characteristics are not identical. In such a case, highly cost switching elements are required to compensate for large loss of the switching elements. Further, the highly cost switching elements lead to a cost increase of the bridge circuit. The above described drawbacks are associated not only with the bridge circuit of the active type vibration control system but also with other bridge circuits for driving the inductance load such as the actuator including the coil.
Accordingly, the above disclosed bridge circuits for driving the inductance load are still susceptible of certain improvements with respect to properly protecting each switching element forming the bridge circuit and supplying a method for driving the bridge circuit in which a stable PWM driving can be carried out.
According to an aspect of the present invention, a method for driving a bridge circuit for driving an inductance load comprises the bridge circuit having a first circuit having a first pair of switching elements arranged in series, a second circuit having a second pair of switching elements arranged in series, a first connecting point for connecting the first circuit and the second circuit, a second connecting point for connecting the first circuit and the second circuit, an inductance load, fly diodes connected to the respective switching elements in parallel. The first connecting point is connected to a power supply and the second connecting point is connected to a ground. The inductance load is connected to an intermediate connecting point between the first pair of switching elements of the first circuit and an intermediate connecting point between the second pair of switching elements of the second circuit. The method for driving the bridge circuit for driving the inductance load in response to a pulse width modulation signal which modulates a control signal with a sine wave by the pulse width modulation turns OFF one of the switching elements on either an upper arm of the bridge circuit connected to the power supply or a lower arm thereof connected to the ground and turns ON the other one of the switching elements on the same arm thereof when the pulse width modulation signal is OFF under the state where a phase difference occurs between an electric current of the control signal with the sine wave and a voltage and where a polarity of the electric current differs from a polarity of the voltage, wherein the electric current is circulated via the other one of the switching elements, the flywheel diode connected to the one of the switching elements, and the inductance load.
According to another aspect of the present invention, the method for driving the bridge circuit for driving the inductance load in response to the pulse width modulation signal which modulates the control signal with the sine wave by the pulse width modulation further comprises turns OFF all the switching elements when the pulse width modulation signal is ON under the state where the phase difference occurs between the electric current of the control signal with the sine wave and the voltage and where the polarity of the electric current differs from the polarity of the voltage.
According to further aspect of the present invention, the method for driving the bridge circuit for driving the inductance load in response to the pulse width modulation signal which modulates the control signal with the sine wave by the pulse width modulation further comprises a means for correcting a duty of the pulse width modulation signal in response to a forward drop of a voltage of the flywheel diode.